1. Field of the Invention
The present invention concerns a method of transmitting data between communication equipments connected to a communication infrastructure.
2. Description of the Prior Art
Throughout the following description the general term "data" refers to speech data or non-speech data such as data representing coded pictures.
The expression "communication equipment" refers to an equipment connected to the infrastructure by wires or by radio links (in which case the expressions "mobiles" and "mobile equipments" are used) or an interface connecting the infrastructure to an equipment or to another infrastructure.
In a conventional digital mobile radio system speech is transmitted in digitized form at a low bit rate, usually less than 10 kbit/s, by radio links between mobiles and base transceiver stations (BTS), called base stations for short, which provide the interface between the mobiles and a fixed infrastructure. The speech is coded using a format whose characteristics include the transmission bit rate. Each mobile has a speech coder/decoder (codec). The telecommunication infrastructures are also equipped with coders/decoders.
In professional mobile radio (PMR) systems many calls are set up between two mobiles or a group of mobiles via one or more base stations.
In these situations there are two options:
either to decode systematically the signal received at the base station, in which case speech is transmitted in the infrastructure at 64 kbit/s or in analog mode, i.e. in the infrastructure speech is coded with a format whose characteristics include the transmission rate employed or the transmission mode employed, this second format as used in the infrastructure being different from that used by the mobiles (note incidentally that the use of the expression "coder/decoder" to describe the devices used in the infrastructure to change the speech format is a misuse of language and that properly speaking these devices are transcoders), PA1 or not to use any coder/decoders within the infrastructure; if the mobiles involved in a call are all in the same cell associated with a base station, which is a relatively frequent situation in professional mobile radio, the infrastructure then serves simply as a "relay". PA1 determining on the basis of the data format used by each of said equipments between which data is to be transmitted whether it is necessary to use a transcoder to transmit said data, and PA1 if so, activating the transcoder nearest the equipment using said first format so that said data is conveyed in coded form to said second format in the infrastructure and changes from said first format to said second format or vice versa as close as possible to the equipment using said first format.
The former solution, which implies the systematic equipment of all base stations with transcoders, is unsatisfactory in terms of quality, efficiency and security of calls.
As any mobile to mobile call involves at least one transcoding operation, namely decoding from the coding format used by the mobiles to the coding format used in the infrastructure, followed by coding from the format used in the infrastructure to the format used by the mobiles, the quality of the call is degraded. What is more, this transcoding of the speech busies unnecessarily physical lines internal to the infrastructure as the speech is conveyed at 64 kbit/s. Furthermore, this solution cannot provide encrypted end to end communication and so does not offer the facility of secure communications between mobile equipments or between the latter and wired equipments connected to the infrastructure.
The second solution, which regards the network as merely a relay for calls between mobiles when the latter are in the same cell, has the drawback that it does not provide for connection to standard telephone equipments including wired terminals and PABX type equipments.
An object of the present invention is to remedy these drawbacks by proposing a method for transmitting data between communication equipments (mobiles, wired terminals, PABX type equipments, etc) connected to a communication infrastructure which optimally processes calls between equipments using a data coding format identical to that used in the infrastructure and equipments which can use a data coding format different from that used in the infrastructure.